In the related art, for example, as an apparatus which reports a traveling state, there has been known an apparatus in which other vehicles are detected by a millimeter wave radar device, the density of the other vehicles within a predetermined distance of a host vehicle is calculated and, further, using critical density according to the speed of the host vehicle, whether the traveling state of the host vehicle will cause traffic congestion or not is determined and the result reported to a driver (for example, refer to patent document 1).